Luther, commenting on Galatians 5:14, wrote:
Paul is an outstanding interpreter of the commandments of God. For he compresses all of Moses into a very brief summary and shows that in all his laws, which are almost endless, nothing is contained except this very brief word: “You shall love your neighbor as yourself.” Reason, of course, is offended at this stinginess and paucity of words, when it is stated so briefly “Believe in Christ” and “You shall love your neighbor as yourself.” Therefore it despises both the doctrine of faith and the doctrine of truly good works. To those who have faith, however, this stingy and paltry phrase “Believe in Christ” is the power of God (Rom. 1:16), by which they overcome sin, death, and the devil, and obtain salvation. So also serving another person through love seems to reason to mean performing unimportant works such as the following: teaching the erring; comforting the afflicted, encouraging the weak; helping the neighbor in whatever way one can; bearing with his rude manners and impoliteness; putting up with annoyances, labors, and the ingratitude and contempt of men in both church and state; obeying the magistrates; treating one’s parents with respect; being patient in the home with a cranky wife and an unmanageable family, and the like. But believe me, these works are so outstanding and brilliant that the whole world cannot comprehend their usefulness and worth, indeed, it cannot estimate the value of even one tiny truly good work, because it does not measure works or anything else on the basis of the Word of God but on the basis of a reason that is wicked, blind, and foolish.
… Therefore we must battle unremittingly not only against the opinions of our own heart, on which by nature we would rather depend in the matter of salvation than on the Word of God, but also against the false front and saintly appearance of self-chosen works. Thus we shall learn to praise the works that each man performs in his calling – even though in external appearances they appear to be trivial and contemptible – provided that they have been commanded by God, and, on the other hand, to despise the works that reason decided upon without a commandment from God, regardless of how brilliant, important, great, or saintly they seem to be.
Luther, Martin Luther’s Works, volume 27: Lectures on Galatians 56, 57